1. Field of the Invention
The present invention is concerned with catalysts and relates to the use of colloidal, stabilized silica in sol form as an auxiliary for stabilizing and/or increasing the activity of catalysts which show a tendency towards activity-reducing sintering during production and/or use under high-temperature conditions.
b 2. Statement of Related Art
According to Ullmanns Encyklopadie der technischen Chemie, IVth Revised and Extended Edition, 1977, Vol. 13, pages 525-537, the suitability of catalysts for industrial processes is based on the four properties of selectivity, useful life, mechanical strength and, above all, activity. The activity of a catalyst, i.e. the extent to which a catalyst accelerates the speed of conversion of the starting materials, is determined both by the chemical properties of the active components and by the topography of the surface. Activity and surface are proportional to one another, i.e. the larger the specific surface, for example through edges and corners of the crystal form or through pores and pore volumes, the higher the activity of the catalyst. Reductions in the activity of a catalyst in use are often attributable to sintering of the surface. At relatively high temperatures above all, the crystallites coalesce as a result of increasing mobility of the surface units into relatively large crystals of lower surface energy, so that the specific surface decreases.
To prevent sintering, either the active components are applied to the surface of an inert, high-melting substance (support) or so-called stabilizers (structural protectors) are incorporated in the crystalline phase.
Known structural stabilizers are oxides, hydroxides and the salts--thermally decomposing into oxides and hydroxides--of aluminium, titanium, thorium, cerium, chromium, magnesium, calcium, barium, strontium, zinc, manganese, hafnium, lanthanum, zirconium, beryllium and silicon, particularly where the compounds are used in the form of dispersions, gels or sols. By virtue of its chemical inertia and its high melting point, particularly in the form of sols, silica may readily be added to catalysts. Suitable silica sols are any of the commercially available types which may differ in their silicon dioxide content, their stabilization and the particle size of the silicon dioxide particles, cf. U.S. Pat. Nos. 2,574,902; 2,577,484; 2,724,701 and 2,375,738.
Silica sols may be used for catalysts inter alia both as binders and as structural protectors which, occasionally, are also referred to as interspersants or spacers.
Silica sols generally show binder properties when the content of silicon dioxide particles in the catalysts is so high that crosslinking silicon-oxygen superstructures are formed, as in the acrylonitrile catalysts of the Sohio process (U.S. Pat. No. 2,904,580), which contain quantities of 30 to 70% by weight silicon dioxide, expressed as solids.
It is known from U.S. Pat. No. 3,216,954 that silica sols inter alia may also be used to prevent crystal growth and/or agglomeration in mangano-chromia-manganite catalysts. According to the Examples, silica sols are added before co-precipitation in quantities of approximately 30% by weight, expressed as solids and based on catalytically active salts. However, it is clear from the disclosure of the above-cited patent specification that the catalysts produced using silica can only be used when the process temperatures can be effectively controlled and do not exceed 700.degree. C.
According to U.S. Pat. No. 4,199,479, an increase in the activity of hydrogenation catalysts containing zinc and copper oxide is obtained by addition of silica sol in quantities of 1 to 30% by weight and preferably in quantities of 15% by weight, expressed as silicon dioxide and based on the final catalyst. However, these catalysts are also exposed to temperatures of only up to 500.degree. C.